Position estimation of a remote target location, i.e. deriving the position/location of a remote object or location of interest, has always been important in both military and civilian environments. In the civilian world, position estimation of a target location, which may be a location or object far away from a surveyor's position, is used in construction, bridge or dam building, land management etc. With the help of a GPS (“Global Positioning System”) receiver and a range finder, the position estimate of a remote location can usually be derived from the surveyor's own location. To reduce the errors inherent in the equipment and in the operator, multiple measurements or “shots” may be made by the surveyor from various widespread locations. Even with multiple surveyors taking multiple shots from different locations, the final position estimate is still unsatisfactory due to the inherent equipment error in each surveyor's equipment. Worse, when multiple surveyors make the observation, the lack of time synchronization adds error and complication in combining measurements for the final estimation.
In the military world, precise and reliable forward observation of targets is one of the most significant capabilities desired by today's military applications. Conventionally, derivation of a remote target position is performed by extrapolating an estimate of range and azimuth from an estimate of an initial position. Target range and azimuth can easily be derived by the use of a common laser range finder (“LRF”), while the initial position can be derived by maps, databases, or GPS receivers. For example, Rockwell Collins, Inc., Assignee of the present application, has provided integrated handheld units of LRF and GPS equipment, which allow data exchange from the LRF to the GPS for computation purposes. The technology underlying the integration of LRF and GPS units has been presented by Mark W. Johnson and Nicholas F. Russo in a publication, entitled “ENHANCED PRECISION TARGETING USING A PLGR GPS RECEIVER INTEGRATED WITH A LASER RANGE FINDER,” published by Institute of Navigation, 1996. The entire disclosure of the publication (hereinafter “Johnson Paper”) is incorporated by reference as if fully set forth herein.
By applying the LRF-generated range and azimuth to the GPS-derived position, the resulting position has approximately the same accuracy as the autonomously-derived GPS position's circular error probable (“CEP”) with some additional error, primarily in azimuth (due to the inherent inaccuracies of the digital magnetic compass) and secondarily in distance (from the LRF's range estimation). The result is a fairly wide error ellipse of the target position and a large estimate of error (which, while elliptical, is still common to utilize a circular estimate of probable error or “CEP” metric to describe). Such errors, commonly known as measurement errors, are compounded by two common source errors, known as systematic errors and operator errors. Additional measurements (also known as “shots”) or estimates by an observer from the same location would do little to reduce the target position error, since the same error sources and issues are still involved. If, however, the observer obtains additional change of azimuth between each new observation, a reduced overall target position error may be achieved by virtue of the overlap of each new position's error ellipse. To reduce the effect of measurement and common source targeting errors, the Johnson Paper teaches a method of combining sufficiently different azimuth measurements from a single user's two or more observations to reduce the overall targeting estimation error.
Despite the obvious advantages, there is still a practical difficulty in the “single user, multiple observations” approach. That is, the single observer must be able to move from position to position in order to obtain the necessary multiple observations. Such a requirement turns out to be impractical and unsafe, since the forward observer is usually in a concealed location. Any unnecessary movement may both place a military user at higher risk of detection and (for commercial or military users) be simply impossible due to environmental surroundings or other constraints.
Another approach has been to place multiple (two or more) observers capable of sighting a common target and combine the multiple observations. By applying each measurement as an uncorrelated vector, i.e. range and azimuth, from independent positions, any attempt to resolve a single point location often becomes an “eyeballing” task by the overseeing officer to determine a compromised estimate without regard to reducing any important contributing factors, such as correlated common and systemic errors and time of measurements.